Plaster of Paris, moulds

The use of monomer-polymer doughs has been largely confined to the production of dentures. A plaster of Paris mould is first prepared from a supplied impression of the mouth. Polymer powder containing a suitable polymerisation initiator is then mixed with some monomer to form a dough. A portion of the dough is then placed in the mould, which is closed, clamped and heated in boiling water. After polymerisation, which usually takes less than half an hour, the mould is cooled and opened. This technique could also be usefully employed for other applications where only a few numbers-off are required but does not seem to have been exploited. 

A suspension is a mixture which arises when solid particles are mixed optimally in a liquid. The suspended solid particles have a diameter of appr. 200-0.5 nm and the mixture is also called a colloidal dispersion . The liquid is the medium of dispersion. A clay suspension is suitable for the production of so-called hollow, non-rotation symmetrical articles, such as sanitary ware. Until the beginning of the 20th century these products were made by beating the clay into plaster of paris moulds, the so-called dies. Gradually people discovered not only the physically and chemical properties of suspensions but also how to change them and thus the technique of clay moulding developed and complicated shapes could be made. The science of colloid chemistry has been essential here. In the field of technical ceramics the moulding technique is also applied with other raw materials than clay. 

Mould makers prepare moulds for the manufacturing of ware with plaster of Paris. Releasing oils are used to facilitate the extraction of the plaster of Paris moulds from the rubber cases. Irritant contact dermatitis is common because of the prolonged contact with water. Allergic contact dermatitis from additives in the releasing agents has been reported (Roberts et al. 1981 Wilkinson et al. 1990). 

The glass polyalkenoate cement can also be used to replace plaster as a mould in the slip process for pottery. It possesses the same property as plaster of Paris, of causing material to deposit on its surface from slip suspensions. So far this property has not been exploited in the manufacture of pottery. 

This manipulation requires that the mass to he formed be of sufficient tractability to be easily introduced into the cavities of the mould by kneading, and that, when pressed, It be of sufficient tenacity as not to bend or yield upon removal. It must, therefore, acquire consistency, and dry in the mould—a condition which Tenders it absolutely necessary that the moulds he constructed of substances of a porous nature, otherwise the pieces would adhere and lose their shape upon removal. The moulds are, therefore, generally formed of gypsum—plaster of Paris—and sometimes of burned clay, and are of every variety of size and pattern, a complete set of pattarns being required for evory new design, and for every size of the same pattern. Moulds for plates, dishes, and other shallow articles, consist of only one pioco, while for jugs, vases, and more elaborate forms, several pieces or moulds are employed. 

From Salts.—(a) Sulphites, thiosulphates and polythionates readily liberate sulphur dioxide when treated with an acid, or, in the case of the salts of the heavier metals, when merely heated. Calcium sulphite mixed with plaster of Paris and moulded into cubes forms a suitable material for use with sulphuric acid in an automatic gas generator such as Kipp s apparatus.6... 

Plaster of Paris (gypsum plaster) is still the most suitable material for casting moulds, being comparatively cheap and allowing easy manufacture of moulds its... 

Plaster of Paris is employed extensively for the making of moulds for slip-casting in this application, the porosity of the set product is important, since water is removed locally from the casting-slip by the capillary action of the pores. Gypsum products however have many other uses, notably in the cement and building industries, and in dentistry and surgery. 

From this original model a plaster mould is made. The generally consists of a cube of solidified Plaster of Paris that has been formed around the model and then cut in half to leave a hollow section of the required shape. These two halves are then held together with rubber bands or tensioned string etc. Some factories preferred to make their moulds out of clay rather than plaster of Paris as they lasted longer. A plaster mould will only reproduce sharp detail for about fifty casting cycles or so. 

The disadvantages are that large-scale production (Fig. 12) requires many moulds and plenty of room, coupled with the fact that plaster moulds have limited durability, as plaster of Paris erodes/corrodes in water processing. 

Antioch Process. Plaster moulds are produced by pouring an aqueous plaster of Paris slip over a mould, steam treating, allowing to set in the air and oven drying. 

Plaster of Paris. Calcium sulphate hemihydrate, CaS04. /2H20 prepared by heating gypsum (q.v.) at 150-160°C. There are two forms a, produced by dehydrating gypsum in water or saturated steam p, produced in an unsaturated atmosphere. Plaster usually contains both forms. It is used for making moulds (q.v.) in the pottery industry. 

Tooling costs can be extremely low because moulds can be made from plaster of Paris or wood. However, for large multi-cavity moulds, as used in high speed moulding, aluminium is the most likely material to be used because of its good thermal contact. Equipment costs can be quite high because both extruders and forming equipment are necessary. 

The application of the colloidal method is certainly not new to ceramic powder preparation, for example, ancient applications include the cigeing of clays for hand moulding. In addition, colloidal methodology has been applied for more than one hundred years to prepare clay slurries for casting into porous moulds, usually plaster-of-paris, to form thin walled, complex shaped bodies. Colloidal methods are also commonly used to fractionate ceramic abrasive media to obtain a desired narrow particle size distribution, however their use prior to consolidation of a ceramic body to eliminate or reduce in size the common heterogeneities associated with ceramic powders, such as agglomerates and inclusions, is relatively rare. 

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